The properties and applications of polyolefins depend to varying degrees upon the specific features of the catalysts used in their preparation. Specific catalyst compositions, activation conditions, steric and electronic features, and the like, all can factor into the characteristics of the resulting polymer product. Indeed, a multitude of polymer features such as co-monomer incorporation, molecular weight, polydispersity, and long-chain branching, and the related physical properties such as density, modulus, melt properties, tensile features, and optical properties, can all be affected by catalyst design.
In recent years, the use of well-defined soluble catalyst precursors generally has allowed enhanced control over polymer properties, including branching architecture, stereochemistry, and block-copolymer construction. This latter aspect of polymer design, in which both “hard” (semicrystalline or high glass transition temperature) blocks and “soft” (amorphous or low glass transition temperature) blocks are assembled in a polymer chain has been especially challenging. Recent advances in block-copolymer formation have been seen with the use of chain-shuttling agents (CSAs) which can exchange a growing polymer chain between different catalytic sites, such that portions of a single polymer molecule are synthesized by at least two different catalysts. In this manner, block copolymers can be prepared from a common monomer environment by using a mixture of catalysts of different selectivities, such as different stereoselectivities or monomer selectivities. Under the right conditions, efficient chain shuttling can produce a multiblock copolymer that features a random distribution of hard and soft blocks of random length.
Generally, the current collection of chain shuttling agents (CSAs) typically contain only a single effective site for chain growth along each polymer chain. Although these CSAs can be considered to contain multiple sites, for example diethyl zinc contains two zinc-ethyl moieties from which a polymer can be attached, chain shuttling between the CSA and catalyst occurs in one direction on each polymer chain. The use of dual-headed CSAs containing an equal number of zinc-alkyl and zinc-alkdiyl groups can potentially lead to reduced homogeneity in the polymer architecture. Such materials can also lead to a broadening of product molecular weight distribution.
Even with the advent of CSA-dual catalyst combinations in block-copolymer preparation, there remain challenges in tailoring the specific copolymer properties that one desires using this approach. Therefore, it is desirable to develop new chain shuttling agents, methods of making the chain shuttling agents, as well as new CSA-catalyst combinations, that can provide new methods to prepare block-copolymers and copolymers with improved properties.